The present invention relates to a fixing device and an image forming apparatus using the same.
An image forming apparatus using the electrophotographic process has been known as a printer or photocopier. In the image forming apparatus of this type, a sheet is passed through the pressure contact portion (fixing nip portion) of a pair of fixing members constituting a fixing device, and the toner is heated, whereby a toner image is fixed onto the sheet. Since fixing is performed by heat and pressure, the sheet having passed through the fixing nip portion may be ejected with the fixing member being wound with the sheet. Then the sheet may not be separated from the fixing member.
For example, the Unexamined Japanese Patent Application Publication No. 2005-258035 discloses a fixing device and image forming apparatus that are provided with a so-called air separation function where a sheet is separated from the fixing member by air blowing. For example, when sheets are processed on a continuous basis, the temperature on the surface of the fixing roller is raised as compared to the case where only one sheet is processed. This may fail to produce a good image quality. To solve this problem, this fixing device changes the wind speed for separation air in conformity to the temperature of the fixing roller. To put it more specifically, if the temperature of the fixing roller is higher than that in a reference range where the satisfactory image quality can be maintained, the wind speed for separation air is set at a value greater than the reference wind speed. This ensures the temperature on the surface of the fixing roller within the reference range. In the meantime, when the temperature of the fixing roller is lower than the reference range, the wind speed for separation air is set at a level lower than the reference wind speed. This prevents fixing failure from occurring due to the toner temperature failing to increase up to the level required for fixing.
In the fixing device of this type, blowing of separation air by the blowing section is suspended in some cases during the time when there is no need for blowing the separation air. The fixing device is placed under the conditions of high temperature and high humidity due to thermal fixing. Accordingly, the air around the fixing member enters the blowing section, and this may raise the following problem. To put it more specifically, condensation is caused on the blowing section, whereby operation failure or a breakdown of the blowing section may occur. Alternatively, when blowing of separation air has started, condensate is scattered on the fixing member. This may reduce the fixing quality.
The Unexamined Japanese Patent Application Publication No. 2005-258035 discloses the technique of variable control of the wind speed for separation air. However, this wind speed control is intended only to change the wind speed within the range of separating the sheets from one another.
The present invention is intended to solve the aforementioned problems. Thus, one of the objects of the present invention is to prevent the air around a fixing member from entering the blowing section for blowing separation air.